Sexually transmitted infections (STI), including human immunodeficiency infections (HIV), are risks associated with each specific sexual event. Most behavioral research measures STI/HIV risk behaviors by average levels of the behavior that may be influenced by relatively distal causal factors like knowledge, attitudes, behavioral skills and other individual attributes. Thus, models of STI risk and protection are insensitive to the relational and contextual phenomena that are proximally associated with coitus, STI prevention, and infection. The proposed research offers a logical extension of prior research by focus on the psychological, interpersonal, social and behavioral phenomena that occur in temporal conjunction with sex. Psychological phenomena include factors such as beliefs/attitudes, mood, emotional arousal and sexual desire/arousal directly associated with a sexual event. Interpersonal phenomena refer to perceptions about a partner such as partner supportiveness and perceived partner sexual desire/arousal. Social phenomena include contexts such as cohabitation, school, and parties that may structure interpersonal romantic/sexual interactions associated with sexual activity. Behavioral phenomena include non-sexual dyadic behaviors, non-coital sexual behaviors as well as coitus-specific behaviors such as douching, contraception or condom use. The research framework addresses phenomena immediately prior to sexual activity, during sexual activity and immediately after sexual activity to capture the contexts within which sexual activity occurs. Thus, the temporal focus of the research is time of entry into physical proximity of two partners to the time of sexual activity (including coitus) to the time of partners' exit from physical proximity. Subjects will be ethnically diverse adolescent and young adult women and men (ages 15 - 29 at enrollment; N=400), recruited from an urban area with high STI prevalence. Data collection will consist of enrollment evaluation and one 12 weeks (84 days) session of daily reports using electronic diary methods. STI testing will occur at the beginning of the 12-week period, at weekly intervals subsequently, and at the end of the 12-week period. The research is relevant to more detailed understanding of risk related to sexually transmitted infections. If successful, the research could inform interventions to improve use of products such as condoms and microbicides, or learning about ways to help people reduce risk of infection.